The present invention relates to a method of forming a modified layer and a pattern.
Formation of modified layers and patterning are required in processes of manufacturing semiconductors and semiconductor devices such as ICs, LSIs, etc.
One silicon semiconductor device structure comprises a substrate having a silicon surface, a desired two-dimensional pattern formed as a silicon oxide film on the substrate, and a modified layer formed by nitriding on the silicon oxide film in substantially the same shape as the silicon oxide film. It has been recognized by the inventors that such a silicon semiconductor device structure is highly effective in the manufacture of various silicon semiconductor devices such as transistors, light detectors, memories, and the like.
The term "substrate" used herein means a substrate made of bulk silicon, a substrate of bulk silicon having an insulation layer or the like and a thin silicon film on the insulation layer, and a substrate comprising a base of, sapphire, quartz or the like and a silicon layer formed thereon. A surface of a substrate on which a two-dimensional pattern will be formed as a silicon oxide film is referred to as a silicon surface which is made of silicon.
Heretofore, a modified layer is formed in a silicon semiconductor device structure of the type described as shown in FIGS. 12(A) through 12(E) of the accompanying drawings.
In FIG. 12(A), a substrate 10 which comprises a silicon substrate by way of example has an upper silicon surface on which a silicon oxide film 12 is formed in a desired two-dimensional pattern up to a prescribed thickness that normally ranges from 0.1 to 1 .mu.m. The two-dimensional pattern has a shape determined dependent on the type and design of a semiconductor device to be manufactured. The upper silicon surface of the substrate 10 has an area 13 free of the two-dimensional pattern, and no silicon oxide film is formed on the area 13 or a silicon oxide film having a thickness ranging from several tens to several hundreds .ANG.. Thus, the silicon surface of the substrate 10 is exposed or covered with a thin silicon oxide film. The two-dimensional pattern is therefore formed as a relief on the silicon surface by the silicon oxide film.
The structure shown in FIG. 12(A) in which the desired two-dimensional pattern is formed as a relief on the substrate by the silicon oxide film is referred to as a material for fabricating a silicon semiconductor device structure.
Then, as shown in FIG. 12(B), a nitride film 14 is deposited o the entire side of the material where the silicon oxide film 12 is formed by CVD employing thermal energy, light energy, plasma, or the like.
Thereafter, the nitride film 4 is removed from the area 13 so that the nitrided film remains only on the two-dimensional pattern.
The above fabrication process has disadvantages as follows: While the nitride film is selectively removed by patterning technology such as photolithography or the like, it is difficult to align the shape of the nitride film with the shape of the silicon oxide film because of limited positioning accuracy and dimensional accuracy upon pattern formation. Consequently, the nitride film 14 and the silicon oxide film 13 tend to be misaligned in shape as shown in FIGS. 12(C), 12(D), and 12(E), for example. As a result, the process has a poor yield in manufacturing semiconductor devices using such silicon semiconductor device structures.
Pattern formation by selective oxidation of a silicon film, a silicide film, or a multilayer film composed of a silicon film and a silicide film will be described below. For selectively oxidizing a silicon film, a silicide film, or a multilayer film composed of a silicon film and a silicide film to form at least a surface of the film as a desired shape, a nitride film or a nitride film deposited on an oxide film is formed as a mask layer on the silicon film, the silicide film, or the multilayer film, and then the mask layer is etched or otherwise patterned into a desired mask shape. Thereafter, the structure is oxidized by being placed in an oxygen or water vapor atmosphere at a high temperature to oxidize those areas of the silicon film, the silicide film, or the multilayer film which are exposed, i.e., are not covered with the mask layer, thus leaving the silicon film, the silicide film, or the multilayer film in the shape of the mask layer under the mask layer.
More specifically, as shown in FIG. 13(A), a relatively thick oxide film 2 is formed on a substrate 1, and a film 4 such as a silicon film, a silicide film, or a multilayer film composed of a silicon film and a silicide film is formed on the oxide film 2. A mask layer 5 which is patterned into a desired shape is deposited on the film 4.
Then, the structure is oxidized to leave the film 4 in the shape of the mask layer 5 under the mask layer 5. Actually, however, as shown in FIG. 13(B), the shape of the film 4 which remains non-oxidized but is left under the mask layer 5 is made much smaller than the mask layer 5. Denoted at 8 is a film which is formed by the films 2 and 4 that are combined together by oxidation.
Where the film 4 is formed on the relatively thick oxide film 2, therefore, the film 4 which is left upon oxidation under the mask layer 5 is much smaller in size than the mask layer 5. In extreme instances, the film 4 beneath the mask layer 5 at narrow portions thereof may be fully oxidized and may not be left under the mask layer 4.
In FIG. 14(A), an oxide film on a substrate 1 includes a thick portion 2A and a thin portion 2B. When the structure is oxidized, as illustrated in FIG. 14(B), a film 4 such as a silicon film, a silicide film, or a multilayer film composed of a silicon film and a silicide film which is left non-oxidized as films 4A, 4B under respective mask layers 5A, 5B. The film 4A on the thick portion 2A of the oxide film is much smaller than the mask layer 5A, whereas the film 4B on the thin portion 2B that is sufficiently thinner than the film 4 is of substantially the same size and shape as the mask layer 5B.
If the oxide film on the substrate has thickness irregularities or thick and thinner portions, as described above, the effect of selective oxidation varies between such thick and thinner portions. Inasmuch as the size of a silicon film, a silicide film, or a multilayer film composed of silicon and silicide films, which is left under a mask layer after oxidation, varies in size dependent on the thickness of an oxide film on which the silicon film, the silicide film, or the multilayer film is formed, it is difficult to effect simultaneous patterning on the silicon, silicide, or multilayer film over the thick and thin portions of the oxide film.